Facings for earthworks are conventionally relatively thick in order to withstand earth pressures, even when the earth is stabilised, for example by inclusion of stabilising members such as reinforcement strips or grids, anchor systems or soil nails. The pressure of the earth on the facing, while often greatly reduced by interaction with stabilising members in the earth, is usually still sufficiently large to require an earth retaining facing comprising concrete panels of some 14-25cm in thickness or other panels of equivalent strength. Such panels are, however, expensive and there is a demand for a modified system using less expensive panels.
We have found that the pressure on the facing is not uniformly distributed but that the areas of the facing close to the points of attachment to stabilising members tend to carry the greater part of the pressure while at more distant locations the pressure is lower. Thus, in a system in which substantially rectangular abutting facing panels are attached to the ends of rows of embedded stabilising elements, the pressure at the centres of the units is significantly lower than that at the periphery where the stabilising members are attached.
This observation appears to be due to the phenomenon of arching within the earth mass. At the present time this phenomenon has not been fully explained and there are at least three theories of its mode of action. (Karl Terghazi, Theoretical Soil Mechanics, Whiley, p66 et seq). In principle, however, in particulate earth, compressive forces at a point are transferred by shear stresses in the earth to more distant points and the forces involved can be shown to follow an arched path within the earth mass. Where, as in the case of panels attached to stabilising members, the earth is rigidly constrained at a number of relatively close adjacent points, the arched lines of force within the earth emanating from adjacent fixed points join to form complete arches within the mass. These arches serve to retain more rearward earth and have the effect of reducing pressure at the facing at locations distant from the fixed points, e.g. at the centres of the facing panels.
Our calculations, as given in greater detail hereinafter, have shown that although arching reduces the earth pressure on the central area of a rigid panel supported between two rigidly held beams, such forces are still large even at parts on the panel at a significant distance from the rigidly held beams. In contrast, where the rigid panel is replaced by an elastic membrane the earth pressure on the elastic surface is greatly reduced even close to the rigidly held beams, although the pressure on the beams is correspondingly increased. Furthermore, the deformation of the elastic membrane is only of the order of a few millimeters, not greatly different from that of a relatively thin conventional concrete panel. In practice, however, deformations of 1-2 cm might be expected.